Crash resistant aluminum alloy sheet products and method of making same

ABSTRACT

Aluminum sheet products having high strengths and favorable crashworthiness are disclosed. The aluminum alloys include Si, Mg and Mn in controlled amounts which provide high yield strengths while retaining formability and crash resistant properties. The sheet products undergo heat treatment and slow quenching. The sheet products are particularly suitable for use as auto body sheet products.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Provisional Application Ser. No. 60/436,123, filed Dec. 23, 2002.

FIELD OF THE INVENTION

The present invention relates to aluminum alloys, and more particularly relates to aluminum sheet products in which alloy compositions and processing methods are controlled in order to produce improved crash resistance properties.

BACKGROUND OF THE INVENTION

The use of aluminum sheet in automotive applications has generally been limited to Aluminum Association 6xxx alloys (Al—Mg—Si) for outer panels and 5xxx alloys (Al—Mg) for inner panels and structural members. In order to maximize the weight savings potential of aluminum, it is desirable to replace relatively low strength 5xxx alloys in the structure with higher strength 6xxx alloys. However, a shortcoming of existing 6xxx auto body sheet (ABS) alloys is their ability to absorb energy during crash situations. This is generally termed crashworthiness.

Autobody sheet requires a combination of good forming properties along with good strength after paint baking operations. The forming properties require good stretch forming and good bending. This traditionally has been achieved with rapid water quenching from solution heat treat temperatures. However, rapid water quenching often results in distortion, surface irregularities and water staining that are unacceptable for outer auto body applications. Air quenching offers many advantages over water quenching with respect to eliminating quench distortion problems, but air quenching can lead to poor bending performance.

The present invention has been developed in view of the foregoing and to address other deficiencies of the prior art.

SUMMARY OF THE INVENTION

The present invention controls alloy compositions and quench rates to produce aluminum alloy sheet products exhibiting good as-processed formability and shape, and good crashworthiness and strength in the artificially aged condition.

An aspect of the invention is to provide a 6xxx alloy with a desired combination of strength and crashworthiness.

Another aspect of the present invention is to provide a heat treated and slow quenched aluminum alloy sheet comprising from about 0.5 to about 0.7 wt. % Si, from about 0.5 to about 0.7 wt. % Mg, from about 0.1 to about 0.3 wt. % Mn, and the balance Al and incidental impurities.

A further aspect of the present invention is to provide a method of treating an aluminum alloy sheet, the method comprising providing a heat treated aluminum alloy sheet comprising Si, Mg, Mn, and the balance aluminum and incidental impurities, and slow quenching the heat treated aluminum sheet at a rate of less than about 200° F./second.

These and other aspects of the present invention will be more apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an aluminum sheet heat treating and slow quenching process in accordance with an embodiment of the present invention.

FIG. 2 is a graph of temperature versus time for a paint bake treatment.

FIG. 3 is a graph of yield strength versus line speed of a continuous heat treat furnace, illustrating strength properties for two different 6xxx alloys without a slow quench and with a slow quench in accordance with embodiments of the present invention.

FIGS. 4 a and 4 b are graphs of tensile properties versus paint bake time at 185° C. for two different alloys.

FIGS. 5 a-5 c are computer generated illustrations taken from different views of a sample crash box made of alloy 6060 sheet without a slow quench.

FIGS. 6 a-6 c are computer generated illustrations taken from different views of a sample crash box made of alloy 6xxA sheet with a slow quench.

FIGS. 7 a-7 c are computer generated illustrations taken from different views of a sample crash box made of alloy 6060 sheet with a slow quench.

FIGS. 8 a-8 c are computer generated illustrations taken from different views of a sample crash box made of alloy 6xxA sheet without a slow quench.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides aluminum alloy sheet products having favorable crash resistant properties. As used herein, the term “sheet” refers to aluminum alloy products having thicknesses from 0.2 to 6.3 mm. For auto body sheet products, thicknesses of from 0.7 to 3.5 mm are preferred. The aluminum alloy sheet products exhibit favorable crash resistance or crashworthiness properties. For the purpose of this invention, crashworthiness is defined as the ability of a material to absorb energy by plastic deformation without appreciable cracking. The crashworthiness of the sheet products can be quantified by critical fracture strain (CFS).

A preferred process path includes the following steps: casting of an aluminum alloy ingot by conventional or continuous methods; hot rolling; intermediate annealing; cold rolling; solution heat treating; and slow quenching, e.g., air quench or minimum distortion water quench. The steps of solution heat treating and slow quenching preferably occur on a continuous heat treater or temper line. After slow quenching, the sheet may optionally be reheated and coil cooled. The optional cooling step may be performed as an off-line batch process. The steps of solution heat treating and slow quenching, in addition to an optional reheating step, are schematically illustrated in FIG. 1.

In the solution heat treatment step, the aluminum alloy sheet may be run through a continuous heat treater to substantially dissolve soluble phases formed during upstream processing. This process typically involves furnace temperatures of 800 to 1,100° F. at speeds from 20 to 150 feet per minute. The temperature and dwell time in the furnace may be adjusted based upon alloy composition and gauge.

In the slow quenching step, upon exit from the furnace zone of the continuous heat treater, the sheet is quenched at a controlled rate to retain the solute in solid solution. This can be accomplished, for example, with air or minimum distortion water. An aspect of this invention is the use of relatively slow quench rates that minimize sheet distortion while still developing favorable physical properties. As used herein, the term “slow quench” means quenching at a rate of less than about 200° F./second, preferably less than about 100° F./second. Quench rates for air type processes preferably range from about 20 to about 100° F./second, more preferably from about 40 to about 70° F./second. Water quench rates preferably range from 50 to 1,000° F./sec, more preferably from 100 to 200° F./second.

In the optional reheating step, a heating unit may follow the quench unit and any coil handling equipment, preferably just ahead of the coiling equipment on the exit end of the line. The heating unit raises the temperature of the sheet such that an elevated coiling temperature can be achieved. A preferred range of coiling temperatures is from about 130 to about 190° F. In the coil cooling step, the warm coil is allowed to cool slowly, typically as a 5,000 to 50,000 lb. mass of metal. This typically results in cooling rates of from about 0.1 to about 5° F./hour.

In accordance with an embodiment of the present invention, the composition of the aluminum alloy sheet is controlled in order to provide favorable crash resistance properties. The Si and Mg levels are controlled in order to provide high strengths. The Mn level is sufficient to control the grain size of the sheet, particularly during heat treating. Suitable alloys include 6xxx alloys such as 6009, 6060, 6063 and 6005. Typical, preferred and more preferred alloy composition ranges are listed in Table 1. TABLE 1 Alloy Compositions (Wt. %) Si Mg Mn Fe Cu Al Typical  0.5-0.7  0.5-0.7  0.1-0.3 0.35 max 0.20 max balance Preferred 0.56-0.68 0.54-0.66 0.12-0.18 0.15-0.30 0.10 max balance More Preferred 0.58-0.66 0.56-0.64 0.12-0.18 0.15-0.25 0.10 max balance

A particularly preferred Al—Mg—Si—Mn alloy is listed in Table 2. Table 2 lists the preferred 6xxA alloy compositions and a 6060 alloy composition in wt. percentages, with the balance comprising aluminum and incidental impurities. TABLE 2 Aluminum Alloy Sheet Compositions Alloy Si % Fe % Cu % Mn % Mg % 6xxA target 0.62 0.20 — 0.15 0.60 min. 0.58 0.15 — 0.12 0.56 max. 0.66 0.25 0.10 0.18 0.64 6060 target 0.56 0.20 0.075 — 0.55 min. 0.53 0.15 0.05 — 0.52 max. 0.58 0.25 0.10 0.10 0.57

An advantage of the present invention is the improvement in the crashworthiness of the aluminum alloy sheet product, which may be measured by critical fracture strain (CFS) and axial crush tests. Using the typical engineering stress-strain output from a standard r&n tension test, a critical fracture strain can be determined: CFS=−1n(1e _(t,eng)) in which e_(t,eng) represents the total engineering thinning strain. The total engineering thinning strain is a function of e_(m), σ_(m) and σ_(f): ε_(t,eng) =f(e _(m), σ_(m), σ_(f)) where em is the engineering strain at the maximum load; σ_(m) is the engineering stress at the maximum load; and σ_(f) is the engineering stress at the fracture load.

The following engineering assumptions are made in the development of the CFS: strains in the thickness and width directions are the same before the maximum load (P_(max)); the true stress after P_(max) is a constant; and the width strain is constant after P_(max). The total thinning strain at fracture may therefore be determined. In accordance with the present invention, a minimum CFS crashworthiness value of about 15 is preferred, with a value of at least 18 being more preferred.

A typical property comparison for alloys is shown in Table 3. TABLE 3 Alloy Yield Strength Crush Results CFS 5083 145 MPa Good with some cracking on tight folds 18 6060 216 MPa Good with some cracking at geometric constraints 21 6xxA 244 MPa Good with some cracking at tight folds 18

Twelve lots of materials 2.0 mm thick were fabricated. Details of the fabrication are given in Table 4. Prior to hot rolling, each of the cast samples was scalped and preheated at 590° C. for 8 hours followed by 560° C. for 9 hours. The main variables were alloy composition, use of a slow spray quench at an approximate cooling rate of 150° F./second following hot rolling, and the line speed of the continuous heat treat furnace (CHT). The compositions of the two 6xxA and 6060 alloys studied are shown above in Table 2. TABLE 4 Coil 1 Coil 2 Coil 3 Coil 4 STEP 6xxA 6xxA 6060 6060 Hot 1080 × 10 mm 1080 × 8 mm 1080 × 10 mm 1080 × Rolling 8 mm Hot >450° C. >450° C. >450° C. >450° C. Rolling exit t. ° C. Slow YES NO YES NO Quench after hot rolling SHT Cont. 550-570- 550-570- 550-570- 550-570- Furnace 570° C. 570° C. 570° C. 570° C. spray bar spray bar spray bar spray bar Speed 1  7 mt/min  5 mt/min  7 mt/min  5 mt/min Speed 2 11 mt/min  8 mt/min 11 mt/min  8 mt/min Speed 3 15 mt/min 12 mt/min 15 mt/min 12 mt/min

The sheet was evaluated in the as received T4 temper and also after a simulative paint bake treatment at 180° C. (365° F.). FIG. 2 is a temperature-time plot of a thermocoupled sheet sample during the paint bake treatment. TABLE 5 Tensile Properties for Sheet Products in T4 Temper ASTM Test Alloy Slow Speed Test Rm Rp0.2 A Uniform A Coil Quench (m/min) direction (MPa) (MPa) (%) (%) r n 6060 No 12 L 164 95 29 24 0.43 0.225 X 155 94 22 21 0.518 0.221 T 152 88 30 25 0.749 0.215 8 L 184 108 28 23 0.634 0.23 X 182 106 23 22 0.634 0.222 T 178 104 25 19 0.749 0.227 5 L 192 116 26 21 0.749 0.223 X 189 115 27 19 0.518 0.216 T 191 116 26 21 0.685 0.22 6060 Yes 15 L 183 113 28 25 0.595 0.216 X 187 110 25 21 0.518 0.24 T 185 108 25 20 0.749 0.232 11 L 180 111 30 24 0.411 0.219 X 183 109 27 22 0.277 0.216 T 183 112 27 22 0.214 0.216 7 L 187 115 27 21 0.427 0.223 X 186 113 26 21 0.346 0.223 T 188 112 26 20 0.267 0.224 6xxA No 12 L 163 94 28 24 0.629 0.231 X 167 90 23 21 0.634 0.243 T 159 91 25 20 0.629 0.222 8 L 196 109 28 22 0.629 0.246 X 192 109 25 21 0.518 0.24 T 189 105 25 21 0.629 0.247 5 L 209 116 27 22 0.634 0.246 X 204 119 24 20 0.518 0.238 T 200 113 25 18 0.629 0.233 6xxA Yes 15 L 196 118 28 20 0.518 0.228 X 194 118 28 21 0.343 0.224 T 188 114 25 21 0.429 0.215 11 L 210 128 28 22 0.477 0.229 X 209 129 26 22 0.477 0.225 T 203 125 23 18 0.524 0.218 7 L 208 125 26 21 0.682 0.232 X 209 124 26 21 0.687 0.234 T 207 124 24 20 0.525 0.233

The tensile properties of the sheet in the T4 temper are presented in Table 5. There was a slight tendency for the T4 yield strength to decrease with increasing CHT line speed, which is probably indicative of incomplete dissolution of Mg₂Si at the faster line speed. Minor variations in other T4 properties were found.

Guided bend tests using T4 sheet pre-strained 10% show that the slow quench is beneficial to bending of both alloys. Both alloys fabricated using the slow quench withstood the maximum sharp bend. Downflange and hemming tests illustrate that both alloys are flat hem capable.

The sheet r&n tensile properties after the paint bake were measured using 2 inch gage length specimens. Table 6 lists the r&n tensile data. TABLE 6 Tensile Properties for Sheet Products after Paint Bake r&n test data (L - direction after PB CHT Uniform Total Slow LineSpeed YS UTS Elong. Elong. Alloy Quench meters/min (MPa) (MPa) YS/UTS (%) (%) r (avg) n 6060 No  5 avg 237 247 0.96 10.3 15.2 1.0064 0.0850 239 249 10.4 16.1 0.9614 0.0845 238 248 10.4 15.7 0.9839 0.0848 6060 No  8 avg 230 238 0.97 10.1 15.5 0.9403 0.0906 230 239 11.0 15.4 0.9982 0.0910 230 238 10.6 15.5 0.9693 0.0908 6060 No 12 avg 197 206 0.95 9.3 15.3 0.9267 0.0958 192 201 9.7 15.0 0.9657 0.0976 194 204 9.5 15.2 0.9462 0.0967 6xxA Yes  7 avg 236 243 0.97 10.2 17.1 0.7234 0.0897 235 243 10.9 16.2 0.7301 0.0912 236 243 10.6 16.7 0.7268 0.0905 6xxA Yes 11 avg 237 245 0.97 10.3 17.0 0.7626 0.0919 237 244 10.7 18.0 0.7448 0.0913 237 244 10.5 17.5 0.7537 0.0916 6xxA Yes 15 avg 228 235 0.97 10.1 15.6 0.8057 0.0917 228 234 10.5 16.4 0.8491 0.0923 228 234 10.3 16.0 0.8274 0.0920 6060 Yes  7 avg 237 245 0.96 10.0 16.4 0.7026 0.0831 237 246 9.9 14.7 0.7120 0.0830 237 246 10.0 15.6 0.7073 0.0831 6060 Yes 11 avg 239 247 0.97 10.4 15.6 0.6590 0.0822 236 245 10.3 16.9 0.6609 0.0839 238 246 10.4 16.3 0.6600 0.0831 6060 Yes 15 avg 236 245 0.97 10.0 15.2 0.6953 0.0804 237 245 10.1 16.5 0.6594 0.0815 237 245 10.1 15.9 0.6774 0.0810 6xxA No  5 avg 249 257 0.97 9.8 14.8 1.0046 0.0832 248 256 9.9 16.4 0.9965 0.0831 249 256 9.9 15.6 1.0006 0.0832 6xxA No  8 avg 232 241 0.97 10.3 16.3 1.0856 0.0904 231 239 10.3 16.0 1.0917 0.0896 232 240 10.3 16.2 1.0887 0.0900 6xxA No 12 avg 165 176 0.94 10.1 16.4 1.0024 0.1137 172 18 10.3 16.1 0.9479 0.1060 169 2179 10.2 16.3 0.9752 0.1099

FIG. 3 plots yield strength of the alloys as a function of the processing variables. The yield strength has the tendency to decrease at the fastest CHT line speeds due to incomplete dissolution of Mg₂Si. Guinier x-ray data showed the presence of Mg₂Si in the materials processed at the faster line speeds. The influence of line speed on yield strength is most pronounced in the sheet which was processed without using the slow quench.

FIG. 4 a is a graph of Rm, Rp0.2 and A values versus paint bake time at 185° C. for the 6060 sample listed in Table 6 which was subjected to the slow quench and a CHT speed of 11 meters/minute. FIG. 4 b is a similar graph for the 6xxA sample which was likewise subjected to the slow quench and CHT speed of 11 meters/minute.

Crash boxes were assembled having a rectangular cross section measuring 63 mm by 133 mm. Welds or rivets may be used at approximately 1 inch on center with the first and last weld approximately ½ inch from the end. The number of spot welds or rivets specified were 20 per flange. An adhesive sold under the designation Betamate 1494 by Gurit Essex is a one-component toughened epoxy that is applied warm along the side seams of the crash boxes, followed by riveting. A pneumatic heated cartridge gun is used to dispense the adhesive at approximately 40 to 50° C. (104 to 122° F.). The metal components to be joined were also heated to approximately the same temperature to assist in application of the adhesive and improve flow and wettability. The adhesive was applied to warm metal on the flanges just prior to spot welding or riveting. Rivets were installed at the same locations specified for welding. End caps are then welded in place. After assembly, the boxes were paint baked. The paint baked boxes were tested in axial crush. The crush loads and energy absorbed at displacements of 100, 150, and 200 mm is given in Table 7. TABLE 7 Axial Crush Test Results of Paint Baked Samples Crush (after PB) CHT Line Max Mean Slow Speed Load Load Energy Absorbed @ (J) Sample No. Alloy Quench meters/min (kN) (J/mm) 100 mm 150 mm 200 mm 1 6060 No  8 avg 144.7 55.7 5941 8472 11039 144.4 56.5 6011 8470 11184 144.9 52.8 5935 8746 10470 144.7 55.0 5962 8563 10898 2 6xxA Yes 11 avg 156.3 54.9 6380 8364 10877 152.8 58.6 6323 8955 11602 157.0 53.4 6171 8496 10571 155.4 55.6 6291 8605 11017 6xxA Yes 11 avg 162.2 58.4 5932 9146 11563 162.2 51.1 5505 8000 10119 156.9 57.3 6690 9149 11361 160.4 55.6 6042 8765 11014 3 6060 Yes 11 avg 154.6 50.4 6076 7924 9983 141.4 53.8 6205 8449 10670 140.6 51.9 5554 8028 10278 145.5 52.0 5945 8134 10310 6060 Yes 11 avg 149.1 56.4 5771 8977 11170 148.1 55.7 6081 8644 11045 153.1 52.3 6437 8175 10364 150.1 54.8 6096 8599 10860 4 6xxA No  8 avg 140.0 51.3 5752 8257 10156 147.8 55.6 5724 8469 11020 143.8 52.8 5992 8461 10455 143.9 53.2 5823 8396 10544

Computer generated illustrations of the crushed appearance of the boxes are shown in FIGS. 5-8. FIGS. 5 a-c are computer generated illustrations from different view of Sample No. 1 listed in Table 7. FIGS. 6 a-c are computer generated illustrations of Sample No. 2. FIGS. 7 a-c are computer generated illustrations of Sample No. 3. FIGS. 8 a-c are computer generated illustrations of Sample No. 4. There were no significant differences among the quantifiable crush parameters for the samples tested.

The performance of the materials met the goals of a sheet alloy product for use in crash critical applications. The paint baked sheet had yield strengths of about 235 MPa, total elongation of 15% and good static crush performance. The T4 properties indicate acceptable formability.

Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims. 

1. A heat treated and slow quenched aluminum alloy sheet comprising from about 0.5 to about 0.7 wt. % Si, from about 0.5 to about 0.7 wt. % Mg, from about 0.1 to about 0.3 wt. % Mn, and the balance Al and incidental impurities.
 2. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the Si comprises from about 0.58 to about 0.68 wt. %.
 3. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the Si comprises from about 0.60 to about 0.66 wt. %.
 4. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the Mg comprises from about 0.56 to about 0.66 wt. %.
 5. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the Mg comprises from about 0.58 to about 0.64 wt. %.
 6. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the Mn comprises from about 0.12 to about 0.18 wt. %.
 7. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the aluminum alloy comprises a maximum of 0.35 wt. % Fe.
 8. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the aluminum alloy comprises from about 0.15 to about 0.30 wt. % Fe.
 9. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the aluminum alloy comprises from about 0.15 to about 0.25 wt. % Fe.
 10. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the aluminum alloy comprises a maximum of 0.20 wt. % Cu.
 11. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the aluminum alloy comprises a maximum of 0.10 wt. % Cu.
 12. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the Si comprises from about 0.58 to about 0.68 wt. %, the Mg comprises from about 0.56 to about 0.66 wt. %, and the Mn comprises from about 0.12 to about 0.18 wt. %.
 13. The heat treated and slow quenched aluminum alloy sheet of claim 12 wherein the aluminum alloy comprises from about 0.15 to about 0.30 wt. % Fe. and a maximum of 0.10 wt. % Cu.
 14. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the Si comprises from about 0.60 to about 0.66 wt. %, the Mg comprises from about 0.58 to about 0.64 wt. %, and the Mn comprises from about 0.12 to about 0.18 wt. %.
 15. The heat treated and slow quenched aluminum alloy sheet of claim 14 wherein the aluminum alloy comprises from about 0.15 to about 0.25 wt. % Fe, and a maximum of 0.10 wt. % Cu.
 16. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the Si comprises about 0.62 wt. %, the Mg comprises 0.60 wt. %, and the Mn comprises about 0.15 wt. %.
 17. The heat treated and slow quenched aluminum alloy sheet of claim 16 wherein the aluminum alloy comprises about 0.20 wt. % Fe.
 18. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has a critical fracture strain of at least
 15. 19. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has a critical fracture strain of at least
 18. 20. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has a yield strength of at least 220 MPa.
 21. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has a yield strength of at least 230 MPa.
 22. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has a yield strength of at least 240 MPa.
 23. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has a critical fracture strain of at least 15, and a yield strength of at least 220 MPa.
 24. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has a critical fracture strain of at least 15, and a yield strength of at least 230 MPa.
 25. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has a critical fracture strain of at least 15, and a yield strength of at least 240 MPa.
 26. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has a critical fracture strain of at least 18, and a yield strength of at least 220 MPa.
 27. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has a critical fracture strain of at least 18, and a yield strength of at least 230 MPa.
 28. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has a critical fracture strain of at least 18, and a yield strength of at least 240 MPa.
 29. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has a thickness of from about 0.7 to about 3.5 mm.
 30. The heat treated and slow quenched aluminum alloy sheet of claim 29 wherein the sheet comprises an auto body sheet.
 31. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has been slow quenched at a rate of less than 200° F./second.
 32. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has been slow quenched at a rate of from about 20 to about 100° F./second.
 33. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has been slow quenched at a rate of from about 40 to about 70° F./second.
 34. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has been slow quenched at a rate of about 150° F./second.
 35. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has been air quenched.
 36. The heat treated and slow quenched aluminum alloy sheet of claim 1 wherein the sheet has further been coil cooled.
 37. The heat treated and slow quenched aluminum alloy sheet of claim 36 wherein the sheet has been coil cooled at a cooling rate of from about 0.1 to about 5° F./hour.
 38. The heat treated and slow quenched aluminum alloy sheet of claim 36 wherein the sheet has been coil cooled from an initial coiling temperature of from about 130 to about 190° F.
 39. A method of treating an aluminum alloy sheet, the method comprising: providing a heat treated aluminum alloy sheet comprising Si, Mg, Mn, and the balance aluminum and incidental impurities; and slow quenching the heat treated aluminum sheet.
 40. The method of claim 39 wherein the sheet is slow quenched at a rate of less than 200° F./second.
 41. The method of claim 39 wherein the sheet is slow quenched at a rate of from about 20 to about 100° F./second.
 42. The method of claim 39 wherein the sheet is slow quenched at a rate of from about 40 to about 70° F./second.
 43. The method of claim 39 wherein the sheet is air quenched.
 44. The method of claim 39 wherein the sheet is coil cooled.
 45. The method of claim 44 wherein the sheet is coil cooled at a cooling rate of from about 0.1 to about 5° F./hour.
 46. The method of claim 44 wherein the sheet is coil cooled from an initial coiling temperature of from about 130 to about 190° F.
 47. The method of claim 39 wherein the aluminum alloy comprises from about 0.5 to about 0.7 wt. % Si, from about 0.5 to about 0.7 wt. % Mg, from about 0.1 to about 0.3 wt. % Mn, and the balance Al and incidental impurities.
 48. The method of claim 47 wherein the Si comprises from about 0.58 to about 0.68 wt. %, the Mg comprises from about 0.56 to about 0.66 wt. %, and the Mn comprises from about 0.12 to about 0.18 wt. %.
 49. The method of claim 48 wherein the aluminum alloy comprises from about 0.15 to about 0.30 wt. % Fe, and a maximum of 0.10 wt. % Cu.
 50. The method of claim 47 wherein the Si comprises from about 0.60 to about 0.66 wt. %, the Mg comprises from about 0.58 to about 0.64 wt. %, and the Mn comprises from about 0.12 to about 0.18 wt. %.
 51. The method of claim 50 wherein the aluminum alloy comprises from about 0.15 to about 0.25 wt. % Fe, and a maximum of 0.10 wt. % Cu.
 52. The method of claim 47 wherein the Si comprises about 0.62 wt. %, the Mg comprises 0.60 wt. %, and the Mn comprises about 0.15 wt. %.
 53. The method of claim 39 wherein the sheet has a critical fracture strain of at least 15, and a yield strength of at least 220 MPa.
 54. The method of claim 39 wherein the sheet has a critical fracture strain of at least 15, and a yield strength of at least 230 MPa.
 55. The method of claim 39 wherein the sheet has a critical fracture strain of at least 15, and a yield strength of at least 240 MPa.
 56. The method of claim 39 wherein the sheet has a critical fracture strain of at least 18, and a yield strength of at least 220 MPa.
 57. The method of claim 39 wherein the sheet has a critical fracture strain of at least 18, and a yield strength of at least 230 MPa.
 58. The method of claim 39 wherein the sheet has a critical fracture strain of at least 18, and a yield strength of at least 240 MPa.
 59. The method of claim 39 wherein the sheet has a thickness of from about 0.7 to about 3.5 mm.
 60. The method of claim 59 wherein the sheet comprises an auto body sheet. 